1. Field of the Invention
The present invention relates to a backup control apparatus. More specifically, the present invention relates to an improvement in a backup control apparatus employed in such as an environment wherein a backup control system is provided in parallel with a main control system for adjusting the water level of a steam generator in a nuclear power plant and the steam generator is controlled by the backup control system by switching the system to the backup control system when the main control system operates in an abnormal state.
2. Description of the Prior Art
It has been a conventional approach that a backup control system is provided in parallel with a main control system for the purpose of enhancing the safety in such an environment as a power plant. In such a case, in switching the system to the backup control system upon occurrence of an abnormal state in the main control system, it is necessary to smoothly switch the line without interruption of a control operation. To that end, it is normally necessary to track the backup control system with respect to the main control system.
FIG. 1 is a block diagram showing one example of a conventional backup control apparatus and FIG. 2 is a block diagram of a switch control portion shown in FIG. 1. The backup control system shown in FIGS. 1 and 2 is an example of adjusting the water level of a steam generator for use in a nuclear power plant. First referring to FIGS. 1 and 2, the structure of a conventional backup control apparatus will be described. For the purpose of adjusting the water level of the steam generator, not shown, the first stage pressure of a steam turbine, not shown, the current water level in the steam generator, the flow rate of water being supplied to the steam generator, and the flow rate of the steam generated by the steam generator are required as parameters. The input signals I1 to I4 representing these parameters are applied in parallel to the main control system 1 and the backup control system 2. The main control system 1 and the backup control system 2 are implemented in substantially the same structure. The main control system 1 comprises a function generator 11, first order lag elements 12, 13, 14, and 15, adder/subtractors 16 and 18, and proportional integrating elements 17 and 19. The function generator 11 is supplied with an input signal I1 representing the pressure value of the steam turbine. The function generator 11 is responsive to the function f(x) to convert the pressure value in proportional to flow rate into a reference water level. More specifically, the function generator 11 evaluates the reference water level based on the pressure value. The reference water level evaluated by the function generator 11 is subjected to mitigation by a first order lag amount 1/(1+TS) by means of a first order lag element 15 for the purpose of avoiding an abrupt change, whereupon the output thereof is applied to an adder/subtractor 16. The input signal I2 representing the value of the water level of the steam generator is applied through the first order lag element 12 to the adder/subtractor 16. Accordingly, the adder/subtractor 16 makes addition/subtraction of the reference water level based on the pressure value and the water level of the steam generator, whereby the output is applied to the proportional integrating element 17. The output from the proportional integrating element 17 is applied to the adder/subtractor 18. The input signal I3 representing a water supply amount is applied to the adder/subtractor 18 through the first order lag element 13 and the input signal I4 representing the flow out amount of the steam from the steam generator is applied through the first order lag element 14 to the adder/subtractor 18. The output of the adder/subtractor 18 is applied to one contact of the switch portion 4 through the proportional integrating element 19.
The backup control system 2 is similar to the main control system 1 and comprises a function generator 21, first order lag elements 22, 23, 24 and 25, adder/subtractors 26 and 28, and proportional integrating elements 27 and 29. The output signal of the backup control system 2 is applied to the other contact of the switch portion 4. Meanwhile, the proportional integrated output signals obtained from the proportional integrating elements 17 and 19 included in the main control system 1 are applied to the proportional integrating elements 27 and 29, respectively, included in the backup control system 2 as a tracking signal. Likewise, the proportional integrated output signals obtained from the proportional integrating elements 27 and 29 of the backup control system 2 are applied to the proportional integrating elements 17 and 19, respectively, of the main control system 1 as a tracking signal.
If and when either of the main control system 1 and the backup control system 2 operates in an abnormal state, the abnormal state is determined by the switch control portion 3 and the switch portion 4 is turned to the output side of the control system in a normal state. The control output signal as switched by the switch portion 4 is applied to the drive portion 5. The drive portion 5 comprises a current/air pressure converter 51, a positioner 52, a booster 53, a diaphragm 54, and a valve 55. The current/air pressure converter 51 serves to convert the control signal obtained from the main control system 1 or the backup control system 2 through the switch portion 4 into an air pressure. The air pressure converted by the current/air pressure converter 51 is applied to the positioner 52. The positioner 52 detects an opened/closed position of the valve 55, thereby to compare the detected opened/closed position with the air pressure obtained from the current/air pressure converter 51. The positioner 52 provides a control signal to the booster 53 for the opened/closed position of the valve 55 consistent with the air pressure obtained from the current/air pressure converter 51. The booster 53 amplifies the control signal applied to the diaphragm 54, thereby to make the opened/closed position of the valve 55 correspond to the air pressure obtained from the current/pressure converter 51. The switch control portion 3 shown in FIG. 1 comprises a control system abnormality determining portion 31 for determining an abnormal state of the main control system 1 and the backup control system 2, and a drive portion abnormality determining portion 32 for determining an abnormal state of the drive portion 5. The control system abnormality determining portion 31 comprises a simulation comparing portion 311 and a reasonable comparing portion 312. The simulation comparing portion 311 serves to compare the output signal from the function generator 11 of the main control system 1 and the output signal from the function generator 21 of the backup control signal 1, for example, thereby to determine whether the respective outputs are consistent with each other. Meanwhile, the simulation comparing portion 311 further comprises a function for determining whether the output of the first order lag elements 12 to 14, the proportional integrating elements 17 and 19 of the main control system 1 are consistent with the respective outputs from the first order lag elements 22 to 24, the proportional integrating elements 17 and 19 of the backup control system 2. The reasonable comparing portion 312 serves to determine, for example, whether the rate of change of the output value of the turbine applied to the function generator 11 is substantially larger than a predetermined value. The outputs from the simulation comparing portion 311 and the reasonable comparing portion 312 are applied through an AND circuit 33 to one input of an AND circuit 35.
The drive portion abnormality determining portion 32 is similar to the control system abnormality determining portion 31 and comprises a simulation comparing portion 321 and a reasonable comparing portion 322. However, the drive portion 5 comprises only one system and therefore an imitation means, not shown, structured to be imitated to the drive portion 5 is provided, so that the simulation comparing portion 321 may determine whether the drive portion 5 is in an abnormal state as compared with the imitation means. The reasonable comparing portion 322 determines whether the rate of change of the air pressure obtained from the current/air pressure converter 51, for example, included in the drive portion 5 is substantially larger than a predetermined value. The respective output signals from the simulation comparing portion 321 and the reasonable comparing portion 322 are applied through an AND circuit 34 to the other input of the above described AND circuit 35. The output of the AND gate 35 is applied through an AND circuit 36 to the switch portion 4. Accordingly, the switch portion 4 is turned when the control system abnormality determining portion 31 and the drive portion abnormality determining portion 32 included in the switch control portion 3 determine that either the main control system 1 or the backup control system 2 is in an abnormal state. Meanwhile, the switch portion 4 may be structured to be unswitchable in a manual mode.
FIGS. 3A and 3B are graphs showing waveforms of the tracking signal shown in FIG. 1. Now referring to FIGS. 3A and 3B, an operation of the FIG. 1 diagram will be described. Let it be assumed, for example, that the switch portion 4 is turned to the output side of the main control system 1 and the drive portion 5 is controlled in response to the control output signal of the main control system 1. Then the tracking signals are individually supplied from the proportional integrating elements 17 and 19 of the main control system 1 to the proportional integrating elements 27 and 29, respectively, of the backup control system 2 and the tracking signals are supplied from the proportional integrating elements 27 and 29 of the backup control system 2 to the proportional integrating elements 17 and 19, respectively, of the main control system 1. It is further assumed that at the timing t1 shown in FIG. 3A the output of the function generator 11 of the main control system 1 abruptly fluctuates as compared with the output of the function generator 21 of the backup control system 2, for example, whereby the curve to be assumed inherently as shown as the curve S1 in FIG. 3A has become as shown as the solid line S2. This abnormal state is determined by the simulation comparing portion 311 of the switch control portion 3. Accordingly, the switch portion 4 is responsive to the switch control signal obtained from the switch control portion 3 to be turned to the output side of backup control system 2 at the time t2. Since the backup control system 2 is tracking the main control system 1 at that time during a time period of the switch timing t1 to t2, the same provides a control signal which is consistent with the control signal of the main control system 1. Since the conventional backup control apparatus is adapted mutually to track simultaneously between the main control system 1 and the backup control system 2 without any time delay, the conventional apparatus has a feature that a continuous control signal can be applied to the drive portion 5 even if the switch portion 4 is turned to either the output side of the main control system 1 or the output side of the backup control system 2.
However, until before the switch control portion 3 determines that an abnormality took place in the main control system 1, for example, and the switch portion 4 is turned to the output side of the backup control system 2, i.e. during a time period of the timing t1 to t2, a tracking signal abruptly fluctuating as shown in FIG. 3B is supplied from the main control system 1 to the backup control system 2. Therefore, the backup control system 2 follows such abnormal tracking signal. As a result, a disturbance that is more than necessary is caused in the backup control system 2 and the drive portion 5 impacts upon an apparatus being controlled, with the result that a stable control cannot be performed.